Kites

ABSTRACT

A box kite employs narrow lifting bands producing improved flight performance and flight stability, and lighter structural loading, permitting efficient use of round-sectioned body members and channel fittings achieving a simple yielding structure which prevents breakage and is inexpensively manufactured and easily assembled and disassembled. The construction is suitable for modular arrangement having several individual kites attached abreast.

United States Patent 1 1 Holland, Jr.

[ 1 Nov. 16, 1973 54] KITES [76] Inventor: Raymond Prunty Holland, Jr.,1702 West Third Street, Roswell, N. Mex.

[22] Filed: Nov. 27, 1970 21 Appl. No.: 93,277

[52] US. Cl. ..244/153 R [51] Int. Cl ..'..B64c 31/06 [58] Field ofSearch ..244/153 R, 154, 155 R [56] References Cited UNITED STATESPATENTS 607,129 Potter ..244/153 R 2,057,148 10/1936 Hopkins ct al...244/l53 R 1328,14 l/l920 Fcrgusson ...244/l53 R 3,494,578 2/1970Coreton ..244/153 R FOREIGN PATENTS OR APPLlCATlONS 555,542 8/1943 GreatBritain ..244/153 R l/1910 France ..244/153 R 11/1905 Great Britain.....

4/1914 France ..244/153R OTHER PUBLICATIONS J. B. Millet ScientificKite-Flying. The Century Magazine Vol. LlV, 1896. Pages 70-72, 77.

Primary Examiner-Milton Buchler Assistant Examiner-Paul E. Sauberer [57]ABSTRACT A box kite employs narrow lifting bands producing improvedflight performance and flight stability, and lighter structural loading,permitting efficient 'use'of round sectioned body members and channelfittings achieving a simple yielding structure which prevents breakageand is inexpensively manufactured and easily assembled and disassembled.The construction is suitable for modular arrangement having severalindividual kites attached abreast.

5 Claims, 5 Drawing Figures KITES The present invention relates toimprovements in box kites and has particular reference to thecommercially successful Skyscraper or narrow-band box kite which, ascompared to existing box kites, employs relatively reduced liftingsurface area and yet is capable of flight in both lighter winds andstronger winds, by virtue of its novel aerodynamic form and structuralbenefits derived from that aerodynamic form.

Although the box kite as a general class of kite has been known for 75years, box kites are still imperfect and subject to inventiveimprovement.

Existing box kites employ lifting surface bands which are broad in chordlength, the perpendicular width or chord length of each band beingtypically about onefourth of the overall length of the kite. Theintended purpose of the broad band in box kites has been to provide thekites with sufficient lifting surface area to enable them to supporttheir weight in light winds. Yet these existing box kites experiencedifficulties due to the broad bands: The edges of the broad liftingbands cannot be kept taut unless structure is provided which is eitherrelatively heavy or consists of multiple elements and is thereforerelatively expensive; the kite tends to be unstable, preventingsuccessful launches in gusty wind, the kite typically rushing offsideways upon being launched, often diving into the ground; the kiterequires a bridle to correct its stability and trim problems, and thisbridle requires repeated adjustment by the kite flier without ever fullyproducing the desired results; breakage occurs when the kite strikes theground; and when the launch is successful, the rate of climb tends to besluggish and the angle of flight tends to be low. The broad bands onexisting box kites, held in tension around a frame of parallellongitudinal body members, tend to bend the body members allowing theband edges to slacken. To correct this the body members have been madethick and heavy, requiring weight of structure, consequently defeatingthe intended purpose of the broad band (preventing flight in lightwinds). Sometimes the bands have been provided with doubled or boundedges, or strings have been placed in the edges, or extra bays ofspreaders have been used to support the body members in order to supportthe bands, all of which have added to the complexity and themanufacturing cost of this kite. The attachment of the spreaders to thebody members also has been unsatisfactory. in some box kites a fittingis used, bound to the body member and having a socket to receive the endof the spreader; these are expensive. In other constructions the end ofthe spreader is slotted and fits over the body member; thisconstruction, if made of wood, which is usually necessary to meet pricecompetition, splits the spreader and destroys the kite when it strikesthe ground. The present invention corrects all of these difficulties ina simple construction.

It is the primary object of this invention to produce a simple,inexpensive, high performance box kite which is crash-proof.

In more detail, the objects of this invention include the following:

To produce a box kite of efficient and stable aerodynamic form, whichproduces a relatively large amount of lift per unit of lifting surfacearea, and a relatively large amount of lift per unit of induced drag,

thereby resulting in a relatively fast rate of climb, a relatively highangle of flight and relatively light air loads applied to the kitestructure, thereby producing a relatively light pull on the flyingstring, permitting a relatively thin" light string to be used withoutrisk of breakaway, and by virtue of the thin light string with itsrelatively reduced wind drag, permitting much higher and steeper flightsto be accomplished, the light applied air loads also permittingrelatively light kite structure to be used, thereby saving weight andimproving kite performance in all departments.

-To produce a box kite having a sturdy, economical, lightweight, simplerigid structure which is shock-resistant, yielding or collapsing insteadof breaking upon striking the ground, and which avoids extra bays ofspreaders and/or reinforcements in the edges of the bands such as arecommon in existing box kites.

To produce a box kite which requires no bridle, nor any adjustment bythe kite flier, and which flies correctly with proper stability and trimevery time it is launched in ordinary winds.

To produce a kite which, by virtue of these improvements, is able to flyin a wind which may be either lighter, or stronger, or gustier than thelightest, or the strongest, or the gustiest wind, in which any simplebox kite could fly before this invention.

To produce a box kite which is easily assembled and disassembled forpackaging, carrying, and flying.

To produce a box kite, which, by virtue of its simplicity and margins ofstrength, rigidity, and flight performance is suitable for modularconstruction, employing several kites flown together attached abreast.

Referring now to the drawing,

FIG. 1 shows at the left the typical proportions of existing box kites1, compared to the box kite 2 of this invention, at the right, with itsnarrow chord lifting surfaces and reduced lifting surface area.

FIG. 2 shows a perspective view from slightly beneath and to one side ofthe box kite 2 of this invention, having strut-like circular-sectionedlongitudinal body members 3; closed band lifting surfaces members 4,made of tough cloth-like flexible sheet material, at-

tached to body members 3; fittings 5, each having a first channel-shapedrecess engaging a member 3, and a second channel-shaped recess oppositethe first recess engaging a strut-like spreader member; spreader members6, engaging in the second recess on fitting 5, just described; andflying line 7. I

FIG. 3 is a view taken at 33 of FIG. 2 showing the manner in whichlongitudinal member 3 and spreader member 6 fit into the recessedchannels in fitting 5. The first recess in fitting 5 receives member 3along the channel, producing bearing contact of significant lengthbetween fitting 5 and member 3, fitting 5 thereby supporting member 3against bending moments caused by the end thrust of spreader member 6and the tension in the material of band 4. Fitting 5, if forced, mayrotate around the center of curvature of circular sectioned member 3,carrying the end of spreader 6 with it, bending spreader 6. If thebending becomes large, spreader 6 will pop out of the second recess infitting 5 before spreader 6 breaks.

FIG. 4 is a view at 4-4 of FIG. 3 indicating how spreader 6, if forced,may move in the second recess of fitting 5, either by rotation or bysliding bodily in the channel, as indicated by the dotted lines. Ifabnormally severe loads occur displacing member 6 it will disengage fromthe channel in fitting before fracture occurs in any part. v

The assembly and disassembly of the kite may be seen from FIGS. 2, 3 and4 to be simple and convenient. In the knocked down form the body members3 remain permanently attached to bands4, in which form the unsupportedmaterial of bands 4 is readily rolled around the longitudinal members 3.For assembly, the kite is unrolled and fittings 5 are pressed onlongitudinal members 3, and the ends of Spreaders 6 are pressed in-thesecond recesses on fittings 5 as shown. The movements which fittings 5may make relative to round section member 3 consisting of rotational andsliding movements, and which spreaders 6 may make relative to fittings5, as illustrated in FIGS. 3 and 4, are of assistance in gettingtautness producing spreader 6 into place. Member 6 may be bent orsloping in being put in place, after which, by means of the deformablestructure described, it may be straightened and squared up to bringlifting surfaces 4 to full tautness. Friction forces are sufficient tohold these parts in place and preserve tautness in flight, but are notsufficient to hold the parts rigidly in a crash loading; the automaticdisengagement of one or more of members 6 from fitting 5 or fitting 5from member 3 prevents breakage of any part.

The chord length of lifting surface 4 is the perpendicular distancebetween the leading edge 9 of this surface and the trailing edge 10 ofthe surface. It is this dimension which is small on the kite of thisinvention relative to the overall length of the kite, producingresultsfdirectly and indirectly, which have heretofore been unknown tokite builders and which would seem to be contrary to common sense inthat the kite of this invention uses reduced lifting area to be able tofly better.

FIG. 5 shows multiple modular box kites made up of four units and twounits of box kite 2, respectively, bound together abreast at adjoininglongitunal members 3 including short portions of these members 8, whichextend forward and rearward of the most forward and most rearward partsof lifting surfaces 4.

To understand the apparent anomaly that the overall flying ability isimproved by reducing lifting surface area one must keep in mind theaerodynamic fact that every surface producing lift distorts the patternof airflow behind that lifting surface. When two lifting surfaces followeach other in tandem, the more forward surface turns the flow downwardat the more rearward surface and reduces the lift at that more rearwardsur face. As two lifting surfaces in tandem, each at the same angle tothe remote wind direction, move relatively closervto each other, thislift-reducing 'effect at the rearward surface becomes relatively larger.But if they are moved farther apart, some of the lift otherwise lost isrestored. In other words, when two equally inclined surfaces in tandemare more widely separated fore and aft they produce more total lift thanwhen they are close to each other fore and aft.

According to elementary aerodynamic theory every lifting surfacegenerates a downwash velocity com ponent at that lifting surface, andthis downwash velocity component persists unchanged behind the liftingsurface, regardless of the distance behind the lifting surface. This, initself, would say that a greater rearward separation of the rearwardbehind the forward surface would not restore lift at the rearwardsurface. This apparent contradiction and commonly held belief isexplained by the existence of three aerodynamic effects in addition tosimple theoretical downwash:

l. The local circulation around the forward surface produces downwardlyinclined flow close behind that surface in addition to and distinct fromthe theorectical downwash velocity component mentioned above. Thisdownward flow due to circulation decreases rapidly as the distancebehind the forward lifting surface is increased, so that lift isregained from this effect as the rearward surface moves fartherrearward.

2. There are three dimensional flow effects which reduce the netdownwash at the downstream lifting surface.v Vortex trunks trail behindthe outboard tips of the forward surface, and the theoretical downwashexists only between these trunks. Outboard, beyond the centers of thesetip vortices, there is no downwash but an upwash instead. Of especialsignificance is the fact that the trailing vortex trunks move laterallytoward each other as they trail rearwardly so that the rearward liftingsurface, when it has the same lateral span dimension as the forwardsurface, as in this case, comes into the upwash flow at its extremeoutboard tips, an effect which generously restores lift at the rearwardsurface when the fore and aft separation of the two surfaces issufficient.

3. Vertical separation of the rearward surface below the forward surfaceenables the rearward surface to escape part of the theoretical downwashfrom the forward surface, producing a progressively increasing liftbenefit at the rearward surface as the rearward surface moves rearwardand downward, as is the case on a box kite.

The primary non-dimensional scale of measurement for comparing theseaerodynamic effects is the chord length of the forward lifting surface;the greater the separation of the forward and rearward lifting surfaceswhen measured in units of such chord lengths, the greater will be thebenefits of this separation.

Hence, the logic of the use of the narrow band on the box kite is this:lt separates the rearward from the forward surface, aerodynamically, andenables the rearward surface to produce lift which it otherwise couldnot produce. This compensates for its smaller size.

Surface area intended to produce lift but located where it cannot do soefficiently, as by broad bands located close behind one another,nevertheless produce drag, a force acting downwind. This drag makes thekite sluggish, prevents high angles of flight, increases structuralloads on the kite's frame and covering, requires heavier structure,increases the load in the flying line, often breaking it, and requires athick heavy flying line which rapidly burdens the kite as it attempts toclimb, principally because of the downward and downwind wind pressure onthe flatly inclined thick string, so that the height which the kite canreach is soon limited. These faults are corrected by the presentinvention.

The Airplane Kite Company performed carefully controlled tests on theevening of May 27, 1970 to investigate the aerodynamic effect of bandwidth on box kites. Two kites were tested, each with longitudinalmembers 2 being 36 inches long and with eachspreader member 6 having alength of 14% inches. The chord length of band 4 was 6 inches on onekite and 4 inches on the other. Both of these kites were of theproportions taught by this patent, their ratios, length/chord being 9 inone case and 6 in the other, whereas this ratio is typically about 4 andoften less, in existing box kites. The test was made to determinewhether the benefit of the narrower chord was still present as the bandchord became very narrow compared to previous practice. The weights ofthe kites were accurately known and they were flown on identical flyinglines, at the same height, side by side in a steady wind which wasobserved to be gradually decreasing in velocity. The flying line wasattached 6 inches behind the leading edge of the front band, in the sameposition on both kites.

The average lifting surface loading of the narrow band kite, totalweight/total lifting area, was 6 percent greater than the loading of thewider band kite.

When the wind velocity decreased sufficiently the first kite to settleto the ground was the wider band kite. The kite with the narrower bandscontinued to fly.

The test was repeated with longer flying lines, allowing the kites toreach stronger winds higher up, sufficient to support them. Once again,as the wind velocity decreased the more lightly loaded kite with thewider bands settled out first, and a short time later as the windvelocity decreased further the narrow band kite came down also. Thesetests showed conclusively that the kite with the very narrow bandsdeveloped a lift coefficient at least 6 percent greater than that of thekite having somewhat wider bands.

The tests also demonstrated visually that the kite with the narrowerbands climbed faster and flew at a steeper angle. Similar differences,but of greater magnitude, are also apparent when narrow band box kitesare flown alongside conventional broad band box kites.

box kite may use members 3 which are circular in section, supported by asingle bay of spreaders.

The circular section of longitudinal member 3 solves the problem ofobtaining suitable sticks for kite manufacture, since hard wood dowelsmay now be used which are available commercially. Thecircular section ofmember 3 also permits the solution of the breakage problem,.in that thejoint between the spreader 6 and member 3 may now be made to yield underload, by the use of fitting 5 which can rotate on member 3. This samefitting 5, conveniently available as a short length of rigid extrusion,also provides the opportunity for a yielding connection relative tospreader 6, furthering the objective of a crashproof structure. Withsuch a construction, the parts can simply turn on each other, or shift,or come apart, instead of breaking when the kite strikes a solid object.

Then, also, fitting 5enables member 3 to be still further reduced insize and weight. The only appreciable load on member 3 occurs where itis in contact with band 4, and fitting 5 is present in this same region,in contact with member 3, supporting it, serving the same function as awing rib. For example, in an extreme form, fitting 5 may be made as longas the chord length of band 4, reaching from the leading edge 9 to thetrailing edge 10 of band 4, thereby removing substantially all of thestress from member 3.

The narrow band box kite, finally, achieves aerodynamic stabilityunknown to the broad band box kite. The narrow chord aerodynamicsurfaces have greater values of the aerodynamic parameter, dC /da, thanbroad chord surfaces; the relative increase of this quantity isespecially great at the rearward surfaces in the presence of the forwardsurfaces, in flight. As a result, aerodynamic damping in pitch and yaware These aerodynamic advantages of the narrow lifting V band are thekey that unlocks a chain of related benefits which are not available tothe conventional broad band box kite.

A narrow lifting band in tension requires less weight of structure forits support than does a broad lifting band. The principal load thatdetermines how large longitudinal member 3 must be is the tension inband 4. Leading edge 9 and trailing edge 10 of band 4 must be keptreasonably taut. When band 4 is narrow, leading edge 9 and trailing edge10 are close together and the bending moment which they exert onlongitudinal member 3 is correspondingly small. As a result, member 3may be smaller and lighter, with substantial weight savings for thekite, or member 3 may be made longer without any net weight increase.

The total air loads on the kite become smaller as band 4 is narrower,because of the drag-producing surface area is reduced. As a result, allstructural members and flying string may be made lighter, and a kitehaving the required structural strength and rigidity to fly in strongwinds, now being lighter, can fly in lighter winds also.

Whereas broad band box kites need members 3 which are either of deepcross section or are supported by more than one bay of spreaders 6, thenarrow band greatly increased for the narrow band box kite as comparedto the broad band box kite. Except in the strongest and gustiest winds,the narrow band box kite can be launched without risk of diving to theground.

The best point for attaching the flying line to the narrow chord boxkite of this invention lies just rearward of the forward band, where itmay be attached directly to member 3, without a bridle. On the broadband box kite, even if the stability were good enough to permit a singletie point to be used, that tie point would lie in the region of the rearportion of the forward band, an inconvenient place to tie the line,because it would have to pass through the material of the band. Ofcourse, because of the difficulty in making the broad band box kite trimproperly in pitch, a branched bridle is ordinarily used, and thisrequires proper adjustment of the lengths of the two branches of thebridle, a complexity which often combines with the marginal stabilityand marginal damping of the kite to frustrate the kite flier.

On. the kite of this invention, on the other hand, the attachment of thestring is so simple and the flight behavior so reliable, that thesenarrow chord box kites may be lashed together abreast by twos or fours,with the flying line attached without a bridle, as shown in FIG. 5, andthey exhibit good stability, impressive high angles of flight, andbecause of their efficient low drag aerodynamic form, only moderatestring pull. The unique structural design described above gives thesekites the structural margins which preserve structural integrity inthese configurations.

I claim:

1. A box kite comprising a longitudinal body member, a lifting surfaceof sheet material in the form of a closed band attached to said bodymember, a fitting having a straight extrusion-like form withsubstantially constant cross-section, said fitting comprising twostraight channel-shaped recesses opening oppositely from each other, thefirst of said recesses engaging said body member in the region ofattachment of said lifting surface, and a spreader member engaging saidfitting in the second of said recesses in said fitting.

2. In claim 1 said channel-shaped recesses in said fitting beingunobstructed fore and aft, whereby said body member and said spreadermember are movable forward and rearward in said recesses, permittingconvenient assembly and producing tautness, as described.

3. In claim 1, said body member having an overall length at least fivetimes as great as the mean chord length of said lifting surface.

4. In claim 1 said body member comprising a short portion extendingforward of the leading edge of the most forward lifting surface on saidkite and a short portion extending rearward of the trailing edge of themost rearward lifting surface on said kite, said kite comprising twomodular units adjoined abreast, bound

1. A box kite comprising a longitudinal body member, a lifting surfaceof sheet material in the form of a closed band attached to said bodymember, a fitting having a straight extrusion-like form withsubstantially constant cross-section, said fitting comprising twostraight channel-shaped recesses opening oppositely from each other, thefirst of said recesses engaging said body member in the region ofattachment of said lifting surface, and a spreader member engaging saidfitting in the second of said recesses in said fitting.
 2. In claim 1said channel-shaped recesses in said fitting being unobstructed fore andaft, whereby said body member and said spreader member are movableforward and rearward in said recesses, permitting convenient assemblyand producing tautness, as described.
 3. In claim 1, said body memberhaving an overall length at least five times as great as the mean chordlength of said lifting surface.
 4. In claim 1 said body membercomprising a short portion extending forward of the leading edge of themost forward lifting surface on said kite and a short portion extendingrearward of the trailing edge of the most rearward lifting surface onsaid kite, said kite comprising two modular units adjoined abreast,bound together at said short portions of said body members, each of saidmodular units being an individual kite capable of flight as a separatekite.
 5. In claim 2, said body member being of circular cross section inthe region of contact with said fitting member within said firstchannel-shaped recess, whereby said fitting is movable in rotationaround the longitudinal axis of said body member, and is movable todisengage from said body member upon the removal of said spreadermember, thereby relieving destructive stresses on impact, as described.